Janet Greenlaw was following her morning routine, checking on the chickens, when she noticed what she thought was a potato resting among the other eggs.

What she thought was a potato turned out to be an egg, twice the size of the others, but that was just the start.

Large eggs aren’t unheard of from nesting fowl like domesticated chickens. Sometimes an extra layer of shell coating might be applied, causing the egg to appear bigger.

Sometimes eggs come out with multiple yolks in one shell. That’s what Greenlaw anticipated when she saw the abnormally large egg that morning.

“I thought it would have two yolks, maybe even three yolks,” Greenlaw said.

She said she was excited, as their chickens had never produced an egg with three yolks before.

Yet even that fell short of the actual outcome. When Greenlaw eventually cracked open the egg, there was only one yolk; however, sitting right next to the yolk, nested like a matryoshka doll, was another fully formed egg.

Greenlaw cracked the inside egg open and found a yolk inside there as well. So the egg did, in fact, have two yolks, the second yolk just happened to be resting inside a whole other egg.

Greenlaw posted a picture of the egg on Facebook and called her grandparents, who had been raising chickens for 80 years.

They’d never heard of anything like it.

By the time she got back to Facebook, a conversation had exploded around the surprising little egg.

Tony Williams, professor of ornithology at Simon Fraser University, is an expert on avian reproduction. Williams said he had never heard of anything like this happening before.

“That’s almost impossible to see how that’s happened,” Williams said.

Because the yolking process is separate from the shelling process, Williams said this sort of thing would be highly improbable.

The only solution Williams could think of, involved the egg being shelled and then staying in the shell gland instead of being released, then when the other yolk comes down, a new shell is produced around both the new yolk and the fully formed egg.

In 2009, an estimated 62.1 million metric tons of eggs were produced worldwide by some 6.4 billion hens.

With that many hens laying that many eggs, phenomena like double, and triple yolked eggs aren’t unheard of, but at least for now, the egg within an egg is a phenomenon rare enough to surprise even experts.